Intelligent sensor method and apparatus for an optical wheel alignment machine

ABSTRACT

A sensor method and apparatus for an optical wheel alignment machine utilizes one or more light sources, such as lasers, to project a laser line or other shaped light onto various locations about the sidewall of a tire undergoing measurement. The sensor includes a video camera or other light responsive receiver and a optical system that combines the reflected laser lines into a single image that is received by the camera. The optical system also rotates one or more of the reflected laser lines so that all of the reflected portions have the same general orientation upon entering the camera. The camera outputs a video data stream that is indicative of the image. The sensor has an electronic circuit that analyzes this video data stream in real time to determine the location in the image of a preselected feature of each of the laser lines. The circuit then outputs coordinate data indicative of the location of this feature. 
     The circuit is microprocessor-based and uses a comparator that compares the incoming video data stream with a threshold to determine, on a pixel-by-pixel basis, whether any particular pixel lies on or outside one of the laser lines received by the camera. For a pixel lying on a laser line, the comparator interrupts operation of the microprocessor which then determines the position within the image of that pixel. Once the pixel representing the preselected feature is found, the microprocessor uses that information to generate the coordinate output data.

TECHNICAL FIELD

This invention relates generally to automobile wheel alignment machinesand, more particularly, to optical sensors used in non-contact wheelalignment machines to determine the alignment characteristics of one ormore of the automobile's wheels.

BACKGROUND OF THE INVENTION

Wheel alignment machines are used by automobile manufacturers andservice centers to determine and set various alignment characteristicsof the vehicle wheels in accordance with the manufacturer's recommendedspecifications. These specifications are selected to meet therequirements of a given vehicle model and may therefore vary from onemodel to the next. Initially, the vehicle wheels are aligned by themanufacturer in accordance with the design specifications. Thereafter,the vehicle wheels should be periodically checked and realigned over theuseful life of the vehicle. Such continued compliance with themanufacturer's recommended specifications is important in insuringproper road handling and minimizing tire wear.

For steerable wheels (e.g., the front two vehicle wheels), there areseveral wheel alignment characteristics that affect steering performanceand tire wear. These characteristics include camber, caster, toe, andsteering axis inclination (SAI). Two of these, caster and SAI angle,relate to the inclination of the steering axis from a purely verticalline and may be determined in various ways known to those skilled in theart. See, for example, U.S. Pat. No. 5,291,660, issued Mar. 8, 1994 toA. Koerner. The other two of these, camber and toe, relate to theorientation of the rotational plane of the vehicle wheel and areimportant to both steerable and unsteerable wheels. In particular, thecamber of a vehicle wheel is a measurement of the inward or outward tiltof the wheel relative to a vertical plane extending in the vehicle'slongitudinal direction. The toe of a vehicle wheel is a measurement ofthe inward or outward tilt of the wheel relative to a horizontal planeextending through the wheel's center.

Many techniques have been developed to determine the rotational planeand, hence, the camber and toe of a vehicle wheel. The vast majority ofthese techniques have required some type of physical contact with thevehicle wheel. For example, in some wheel alignment machines, positionencoders are used to generate signals indicative of the positions ofrollers that contact the tire's sidewall. Generally, three such rollersand associated encoders are used for each wheel, with the rollers beingspaced ninety degrees apart. The encoders provide a data streamindicating the distances of the rollers from a vertically disposed planethat extends in the vehicle's longitudinal direction. This data is usedby a central computer to calculate the camber and toe angles.

More recently, optical techniques have been developed that permitdetermination of these alignment characteristics without any targets orother parts of the measurement apparatus having to come into physicalcontact with the vehicle wheel. See, for example, U.S. Pat. No.4,745,469, issued May 17, 1988 to T. J. Waldecker et al.; U.S. Pat. No.4,863,266, issued Sep. 5, 1989 to S. Masuko et al.; and U.S. Pat. No.5,268,731, issued Dec. 7, 1993 to M. Fuchiwaki et al. Non-contactoptical measurement of wheel alignment characteristics is advantageousbecause it provides good resolution and the ability to determinealignment characteristics without moving parts and without requiringcontact between the measurement equipment and the vehicle. For eachwheel, actual measurement of the wheel position is accomplished using anoptical sensor that includes a light source for projecting light ontothe wheel and a light responsive receiver for sensing a portion of theprojected light reflected off the wheel.

In the patent to Waldecker et al., wheel alignment measurement isaccomplished using a non-contact sensor station located adjacent each ofthe vehicle wheels. The sensor station for each wheel utilizes aplurality of sensor modules, each of which includes a laser light sourceand an associated video camera. The laser light sources at each wheelare used to project stripes of laser light that extend radially acrossthe sidewall at two or more spaced locations. The video cameras at eachsensor station are used to sense reflections of their associated laser'slight off the tire's sidewall. Each camera is offset from the opticalplane of its associated laser to give it a perspective view that permitsthe determination of distance between the sensor and tire sidewall.Because of the curved contour of the tire's sidewall, the reflected lineof laser light, as seen from the perspective of the associated videocamera, will have a generally parabolic shape. The images sensed by thevideo cameras are provided to a computer system that includes analignment processor engine and an integrated host/alignment processorengine, both of which employ separate VME buses for communication. Theseprocessor engines are used to analyze the images to determine thelocations within the images of the reflected laser lines. Thisinformation is then used to derive the camber, toe, or both. The systemcan include a transition detection circuit that monitors the digitalvideo stream as it is being written into memory to determine the crownof the reflected parabolic laser line, which represents the point on thetire sidewall that is closest to the sensor module. This is accomplishedby turning each camera on its side such that the reflected laser line,as seen by the camera, has a generally vertical orientation. Each scanline of the camera will then intersect the reflected laser line and theclosest point of the tire sidewall can then be found by determining therow (scan line) number and column number of the crown of the reflectedparabolic laser line. This information can then be used by theprocessors to quickly determine the probable location of the closestpoint to the sensor module.

This prior art system suffers primarily from its complexity and theresulting cost to implement. More specifically, in the preferredembodiment, the system utilizes three lasers and three video cameras foreach wheel to determine the camber and toe angles. For a vehicle havingfour wheels, this means twelve lasers and twelve video cameras arerequired. Additionally, the computer system acquires complete frames ofdata from each of these cameras and must then utilize the two VME basedprocessors operating in parallel to perform image processing todetermine the location within each frame of the reflected laser light.Furthermore, the sensor stations do not themselves provide dataindicative of the distance between the tire and sensor; rather, theyonly provide video frames and, therefore, their output data cannot beused with existing wheel alignment computers or in a conventional mannerto determine the wheel alignment characteristics.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided anintelligent sensor for use in a wheel alignment machine to measure theorientation of a tire on a vehicle to thereby determine one or morealignment characteristics of the vehicle. The sensor includes one ormore light sources oriented to project shaped light onto a sidewall ofthe tire at a plurality of spaced locations and a video camera or otherlight responsive receiver oriented at a perspective angle with respectto the light source(s) to receive an image that includes a portion ofthe shaped light that is reflected off the tire. The sensor furtherincludes a system of optical elements and an electronic circuit. Theoptical elements are oriented relative to the light responsive receiverto provide the light responsive receiver with an optical view thatincludes the plurality of spaced locations of the tire. Thus, portionsof the shaped light that are reflected off the tire at each of theplurality of spaced locations are received by the light responsivereceiver as a single image. The light responsive receiver generateselectrical signals indicative of this image. The electronic circuitutilizes the electrical signals to determine the location within theimage of a predetermined feature of each of the reflected portions ofshaped light, and the circuit generates output data representative ofthe locations of the predetermined features.

Preferably, the light source(s) are three visible lasers, each of whichis oriented to project a stripe of laser light onto the sidewall at adifferent one three spaced locations, whereby the shaped light at eachof the plurality of spaced locations comprises a stripe of laser light.Also, the light responsive receiver preferably comprises a video camerahaving an image receiving element that includes successive scan lines,each of which comprises a number of pixels.

In accordance with another aspect of the invention, the sensor operatesto determine a preselected feature of each of the reflected portions oflaser lines or other shaped light in real time and to generateconventional output coordinate data for use by a computer to determineone or more alignment characteristics. For this purpose, the opticalelements are preferably oriented to rotate at least one of the reflectedlaser lines with respect to at least one other of the reflected laserlines so that all three reflected laser lines have the same orientationwhen they enter the light responsive receiver. The video cameragenerates the electrical signals as a stream of pixel data pointsarranged into successive lines of the pixel data points, with each ofthe lines of pixel data points representing one of the scan lines. It isoriented to receive the reflected portions of the stripes of laser lightas lines of laser light that intersect at least some of the camera'sscan lines. The circuit includes a microprocessor and operates tomonitor the stream of pixel data points as it is received from the videocamera. The electronic circuit interrupts operation of themicroprocessor when the circuit receives a particular pixel data pointrepresentative of any of the reflected portions of the stripes of laserlight. The microprocessor responds by acquiring a pixel countrepresenting the position of the particular pixel data point within itsassociated line of pixel data points. Using this pixel count and theknown position of the scan line in the video frame, the circuitgenerates output data representative of the coordinates of a preselectedfeature of the reflected laser lines.

By combining the reflected laser lines or other portions of shaped lightinto a single image, only one video camera or other light responsivereceiver is needed. Also, by rotating one or more of the reflectedportions of shaped light so that they all have the same orientation, andby providing them to the camera such that they intersect the camera'sscan lines, real-time analysis of the video data to determineconventional coordinate data indicative of the wheel's rotational planeis made possible.

In accordance with yet another aspect of the invention, a method isprovided for generating data indicative of the orientation of a tire ona vehicle, comprising the steps of:

(a) projecting shaped light onto a sidewall of a tire at a plurality ofspaced locations,

(b) receiving an image that includes portions of the shaped lightreflected at an angle off the sidewall from each of the plurality ofspaced locations,

(c) generating a video signal that comprises a stream of pixel intensitylevels arranged into successive groups of the pixel intensity levels,each of the groups representing a row of an array of pixels intensitylevels that together represent the image,

(d) providing a threshold intensity level that is less than those of thepixel intensity levels that represent the reflected portions of theshaped light,

(e) determining a plurality of pixel counts by repeating the followingsteps (e1) through (e3) for each of a plurality of the groups of pixelintensity levels:

(e1) comparing, in real time, the threshold intensity level with atleast some of the pixel intensity levels of one of the groups,

(e2) generating a logic signal when one of the pixel intensity levelsexceeds the threshold intensity level, and

(e3) determining, in response to an occurrence of the logic signal, oneof the pixel counts in accordance with the position of the one of thepixel intensity levels within the one of the groups, and

(f) using the pixel counts to generate output data representative of thespatial position of the plurality of spaced locations.

In accordance with a broader aspect of the invention, theabove-summarized method and apparatus of the invention could be utilizedto determine one or more spatial attributes of an object other than avehicle wheel. Rather than projecting shaped light onto a tire todetermine alignment characteristics, the shaped light can be projectedon any of a number of different types of objects to determine theposition, orientation, or other spatial attribute of the object.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred exemplary embodiment of the present invention willhereinafter be described in conjunction with the appended drawings,wherein like designations denote like elements, and:

FIG. 1 is a diagrammatic overview of a preferred embodiment of anoptical wheel alignment sensor of the present invention;

FIG. 2 is a diagrammatic plan view of a wheel alignment machine thatincludes four of the optical wheel alignment sensors of FIG. 1;

FIG. 3 depicts the convention used to define the orientation of avehicle undergoing measurement in the wheel alignment machine of FIG. 2;

FIG. 4 is a perspective view showing the positioning of one of thesensors in FIG. 2 relative to the left front wheel of the vehicle andshowing the convention used to define the spatial positions of thesensor and wheel for the purposes of measurement of the wheel'salignment characteristics;

FIG. 5 is a side view of the vehicle's left front wheel showingdiagrammatically how lasers within the sensor of FIG. 4 are used toproject shaped laser light onto the sidewall of a tire mounted on thewheel;

FIG. 6 is a top view of the vehicle showing the relative positioning ofthe wheel and sensor of FIG. 4, with the wheel exhibiting positive toe;

FIG. 7 is a front view of the vehicle showing the relative positioningof the wheel and sensor of FIG. 4, with the wheel exhibiting positivecamber;

FIG. 8 is a plan view of the vehicle depicting the effect ofmisalignment of the vehicle with the sensors;

FIG. 9 is a plan view of the wheel and sensor of FIG. 4 showing thesensor optical system;

FIG. 10 shows an image output by the camera used by the sensor of FIG.4, with the image representing the reflected shaped light from threecircumferentially spaced locations on a master gauge used to calibratethe sensor;

FIG. 11 is an enlarged view of one of the reflected portions of shapedlight from FIG. 10 and is included as part of a general description of apreferred procedure for determining the high point of the reflectedportion of shaped light;

FIG. 12A is a side view of the wheel of FIG. 4 showing how the wheel'stoe angle effects the location at which the shaped light hits the tire'ssidewall;

FIG. 12B shows the effect of toe on the relative and absolute locationsof the reflected shaped light within the image generated by the sensorcamera;

FIG. 13A is a side view of the wheel of FIG. 4 showing how the wheel'scamber angle effects the location at which the shaped light hits thetire's sidewall;

FIG. 13B shows the effect of camber on the relative and absolutelocations of the reflected shaped light within the image generated bythe sensor camera;

FIG. 14 is a block diagram of the sensor electronics; and

FIG. 15 is a timing diagram showing various signals used and generatedby the sensor electronics.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Sensor/Wheel Alignment Machine Overview

As shown in FIG. 1, a preferred embodiment of an optical wheel alignmentsensor of the invention, designated generally at 10, utilizes threelasers 12, 13, and 14, an optical system 16, a video camera 18, and anelectronic circuit 20, all of which are contained in a housing 22. Ingeneral, lasers 12-14 are used to project shaped laser light onto three,spaced locations on the sidewall of a tire that is mounted on a wheelundergoing measurement. Diffusely reflected laser light from each of thethree locations re-enters housing 22 and optical system 16 where it isreflected and, in some cases, rotated to combine the three reflectedportions of laser light into a single image that is then directed intocamera 18. Camera 18 outputs a video stream representing successiveframes of images and this output is provided to electronic circuit 20.The video data is analyzed in real time by circuit 20 to generate outputdata representative of the positions within each image of the threereflected portions of laser light.

Referring now also to FIG. 2, this output data is sent from circuit 20to a central computer 24 that forms a part of a wheel alignment machine26 with which sensor 10 is used. Sensor 10 is one of four identicalsensors 10a-10d used as a part of wheel alignment machine 26 todetermine certain alignment characteristics of each of the four wheels28a-28d of a typical automobile 28. Each of the sensors 10a-10d aremounted on a vehicle test rig (not shown) that supports automobile 28 ina conventional manner. The positions of each of the sensors 10a-10d isselected so that each sensor will be adjacent one of the four wheels28a-28d. If desired, sensors 10a-10d can be adjustable in one or moredirections to account for different vehicle wheel bases, vehicle widths,and wheel widths and diameters.

The output data from each of the sensors 10a-10d is sent over adifferent one of four RS-422 serial lines that are connected to a fourchannel serial card 24a within computer 24. These serial lines are alsoused to send commands from computer 24 to one or more of the sensors10a-10d, as will be described. Preferably, each of the sensors alsoprovides a video signal to a four channel video multiplexor 24b withincomputer 24 for display on a monitor 25. For each sensor, this videosignal is generated by circuit 20, which conditions the video signaloutput by its associated camera 18 and superimposes on the image one ormore cursors that are used to highlight a particular feature of each ofthe three reflected portions of laser light.

Spatial Conventions

Before describing the construction and operation of sensor 10 in detail,reference is made to FIGS. 3 and 4 which show certain of the conventionsthat will be used below. As shown in FIG. 3, the orientation of vehicle28 can be depicted by three orthogonally related axes: a longitudinalaxis extending along the length of the vehicle, a lateral axis extendingalong the width of the vehicle, and a vertical axis extending in thethird dimension, with the three axes intersecting at a point arbitrarilyselected to be the center of vehicle 28. As will be discussed furtherbelow, vehicle 28 is preferably oriented on the test rig with itslongitudinal axis aligned with the test rig's longitudinal axis and anydeviations between the two are accounted for by computer 24 using asymmetry correction.

FIG. 4 depicts the relative positioning of sensor 10a relative to theleft, front wheel 28a. In the description that follows, only sensor 10aand its use with wheel 28a will be discussed and it will be understoodthat the discussion will apply equally to the other sensors 10b-10d andtheir use with wheels 28b-28d, respectively. Wheel 28a includes a tire30a having a sidewall 32a upon which the laser light is directed for thepurposes of measurement. Wheel 28a is supported by tire 30a on a pair ofrollers 34, one or both of which can be a driven roller to permitdynamic measurement of the various wheel alignment characteristics.

For the purpose of wheel alignment measurement, sensor 10a and wheel 28aare both spatially referenced to three orthogonally related X, Y, and Zaxes. These axes are parallel to the longitudinal, vertical, and lateralaxes, respectively, of the test rig. The optical path of sensor 10a, asdetermined specifically by optical system 16 and camera 18, is generallyalong the Z-axis and camera 18 therefore receives an image that liesgenerally within the plane defined by the X and Y axes.

Sensor Construction and Operation Overview

Referring back to FIG. 1, lasers 12 and 14 are located near the bottomof housing 12 and laser 13 is located at the top of housing 22. Opticalsystem 16 is located at the top, front portion of housing 22 abovelasers 12 and 14 and just below laser 13. Behind optical system 16 iscamera 18 and at the very rear of housing 22 is electronic circuit 20.Lasers 12-14 each project shaped laser light out of housing 22 and ontosidewall 32a of tire 30a through a respective aperture 22a-22c. Portionsof the laser light reflected off sidewall 32a re-enters housing 22through one of two apertures 22d and 22e. These two apertures arecovered by filters 36 and 38, respectively, to keep out light atwavelengths other than that of the reflected laser light. These filtersprovide camera 18 with a dark background to help maximize the contrastbetween the reflected laser light and its background. Reflected lightoriginating from laser 12 enters through aperture 22d and is redirectedby optical system 16 along an optical path and into camera 18's field ofview. Reflected light from lasers 13 and 14 enters through aperture 22eand is directed into camera 18 along one of two other optical paths,depending upon where on sidewall 32a the light was reflected from. Theconstruction of optical system 16 that provides these three opticalpaths to camera 18 will be described below.

Turning now to FIG. 5, the projection of shaped light by lasers 12-14onto sidewall 32a of tire 30a will be described. As used herein, shapedlight should be understood to mean light, whether coherent or not andwhether focussed, collimated, scanned, diffracted or otherwise, that isprojected onto sidewall 32a in a predetermined conformation. Preferably,lasers 12-14 each emit a plane of visible light that appears on sidewall32a as a radially oriented stripe of laser light. Suitable lasers forproviding a stripe of laser light are commercially available, such as anLG23, available from Applied Laser Systems. As shown, lasers 12 and 14are used to project their laser line onto sidewall 32a at forward andrear locations, respectively, midway up the tire. These two laser linesare used to determine the toe angle for wheel 28a. Laser 13 is used toproject its laser line onto sidewall 32a at the uppermost part of tire30a. The laser line emitted by laser 13 is used in connection with theother two laser lines to determine the camber angle for wheel 28a. Thus,lasers 12 and 14 project in a plane that is parallel to the longitudinalaxis of the test rig (i.e., parallel to the sensor's X-axis) and laser13 projects in a plane that is parallel to the vertical axis (i.e.,parallel to the sensor's Y-axis). The laser line emitted by laser 12 isreferred to as FTOE (front toe) and the laser line emitted by laser 14is referred to as RTOE (rear toe), with these labels indicating theirrelative positions on sidewall 32a. Thus, the FTOE laser line is solabelled because it is located closer to the front end of vehicle 28than RTOE and, similarly, RTOE is so labelled because it is more towardsthe rear end of vehicle 28. The laser line emitted by laser 13 isreferred to as CAM, since it is used in to determine the camber anglefor wheel 28a.

Lasers 12 and 14 are located at the bottom of housing 22 and they aretherefore directed upwards at an angle. Also, since apertures 22d and22e are located near the top of housing 22 and are therefore offset fromlasers 12 and 14, reflected laser light enters through these aperturesat an upwards angle. Preferably, the total angle between the directionof laser light emitted from lasers 12 and 14 and the direction ofreflected laser light received within apertures 22d and 22e isfifty-five degrees. Laser 13 is directed towards the front of vehicle 28at a relatively small angle with respect to the Z-axis shown in FIG. 4and the reflected CAM laser line enters through aperture 22e at a largerangle with respect to this same axis. It will be appreciated that,although the CAM laser line appears from the perspective of the Z-axisas having its concave side facing toward the front of vehicle 28, theCAM laser line enters through aperture 22e with its concave side facingrearwardly. This occurs because aperture 22e sees the reflected CAMlaser line from the right or rearward side of laser 13, whereas theZ-axis perspective shown in FIG. 5 is from the left or forward side oflaser 13. In the illustrated embodiment, laser 13 is at a fifteen degreeangle from the Z-axis, as measured in the X-direction, and the CAM laserlight enters aperture 22e at a thirty degree angle from the Z-axis,giving fifteen degrees of relative angle between the emitted andreflected CAM laser line. Alternatively, laser 13 can be directedstraight along the Z-axis shown in FIG. 4 with the reflected CAM laserlight entering aperture 22e at a forty-five degree angle to the Z-axisto give a greater degree of perspective angle.

In accordance with the normal shape of vehicle tires, sidewall 32a has acurved surface and the reflected FTOE, RTOE, and CAM laser lines have agenerally parabolic shape when viewed from the perspective angles ofoptical system 16. As will be appreciated, the crown, or high-point, ofeach of the curved laser lines represents the point along the laser linethat is closest to sensor 10a. Preferably, lasers 12 and 14 are aimed sothat, for a vehicle properly centered in the test rig and having zerocamber and toe, the high point of the FTOE and RTOE laser lines on thetire sidewall is within the horizontal plane defined by the X and Zaxes. Also, laser 13 is preferably aimed so that, for that same vehicle,the high point of the CAM laser line on the tire sidewall is within thevertical plane defined by the Y and Z axes.

As is known by those skilled in the art, the high point of each of thethree laser lines defines the rotational plane of tire 30a. Since thetoe of wheel 28a is the angle between this rotational plane and thevehicle's longitudinal axis, the difference between the distance z_(f)from FTOE to sensor 10a and the distance z_(r) from RTOE to sensor 10acan be used to determine this toe angle. This is shown in FIG. 6.Similarly, and as shown in FIG. 7, since the camber of wheel 28a is theangle between the wheel's rotational plane and the vehicle's verticalaxis, the difference between a distance z_(d) and the distance z_(c)from CAM to sensor 10a can be used to determine this camber angle.Distance z_(d) is the distance along the Z-axis between sensor 10a andthe center of wheel 28a and is determined in a manner that will bedescribed in greater detail below.

The distances between sensor 10a and the high points of FTOE, RTOE, andCAM (i.e., z_(f), z_(r), and z_(c)) are not measured directly by sensor10a. Rather, sensor 10a determines the X and Y coordinates of these highpoints and sends them to computer 24, which determines the z_(f), z_(r),and z_(c) distances by comparing the coordinates for each high point toknown reference points. As is known, use of these coordinates to derivethe Z-axis distances is possible because changes in the distance betweensensor 10a and sidewall 32a cause changes in the locations on sidewall32a where the laser light hits, resulting in changes in one or both ofthe X and Y coordinates.

Since toe and camber are expressed as angular quantities relative to thevehicle's longitudinal and vertical axes, respectively, deviations ofthese axes from parallelism with those of the test rig (i.e., the X andY axes of sensor 10a, respectively) could result in erroneous toe andcamber measurements. In accordance with conventional practice, alignmentof the vehicle's and test rig's vertical axes is assumed and deviationsbetween the longitudinal axes are accounted for using a symmetrycorrection that is determined and implemented in a conventional mannerby computer 24. If the vehicle is centered in the test rig with its axesaligned with those of the test rig (and, thus, sensors 10a-10d), thedistance z_(d) between the center of each wheel and its associatedsensor will be the same. If not, some or all of the distances z_(d) willbe different. This is illustrated in FIG. 8, which depicts in anexaggerated fashion the effects on z_(d) of misalignment of vehicle 28with sensors 10a-10d. By comparing the distances z_(d) to each other,the symmetry correction needed can be determined and thereafter appliedas a part of determining the alignment characteristics of each of thewheels.

One advantage of using sensors 10a-10d to output the X and Y coordinatesof the high points of the FTOE, RTOE, and CAM laser lines is that theyprovide the same information as prior art wheel alignment sensors thatrequire contact with the sidewall of each tire. Thus, wheel alignmentmachine 26 can utilize conventional techniques for analyzing the sensoroutput to determine the vehicle's alignment characteristics. This isadvantageous because it not only allows for the possibility ofretrofitting the sensors on existing contact-based wheel alignmentmachines with relatively minor modifications, but it permits use ofwidely known and thoroughly understood techniques for determining therotational plane of the wheels, the symmetry correction, and from thesethe wheel alignment characteristics.

Sensor Optical System and Camera

Referring to FIG. 9, optical system 16 will now be described. Ingeneral, optical system 16 utilizes a series of mirrors or otherreflective elements to perform two primary functions; namely, rotatingthe CAM laser line so that it has the same orientation as the FTOE andRTOE laser lines, and thereafter combining all three laser lines into asingle image that is directed into video camera 18. In particular,optical system 16 includes mirrors 42 and 44 which direct the reflectedFTOE laser line into camera 18, mirrors 46 and 48 which direct thereflected RTOE laser line into camera 18, and mirrors 50 and 52 which,along with mirror 48, directs the reflected CAM laser line into camera18. Camera 18 and mirrors 42, 44, 46, and 48 are each rigidly mounted ona support plate (not shown) that is angled downwardly toward the frontof sensor 10a. This gives optical system 16 has a generally downwardperspective. The downward angle of the support plate from the horizon(Z-axis) is the same as the angle of the reflected FTOE and RTOE laserlight so that those two laser lines enter optical system 16 in adirection parallel to the surface of the support plate. Mirror 42reflects the FTOE laser line after it enters housing 22 by ninetydegrees towards mirror 44. Similarly, mirror 46 reflects the RTOE laserline by ninety degrees towards mirror 48. Mirrors 44 and 48 are eachdisposed in a plane parallel to their associated mirrors 42 and 46,respectively, to direct the FTOE and RTOE laser lines into camera 18along paths that are parallel to the paths they took when enteringhousing 22. The back sides of mirrors 44 and 48 are disposed ninetydegrees apart and are in contact with each other along one side thatfaces the lens 18a of camera 18. As shown, mirrors 44 and 48substantially fill the field of view (FOV) of lens 18a.

Mirrors 50 and 52 are rigidly mounted within housing 22 so that theirposition and orientation is fixed with respect to the support plate. Ifdesired, the position of camera 18 and any of the mirrors can of coursebe initially calibrated or made adjustable for periodic calibration inthe field. Mirror 50 reflects the CAM laser line upwards after it entershousing 22 towards mirror 52. This laser line is then reflected towardsmirror 48 which directs it into camera 18. As will be appreciated, theadditional reflection provided by mirrors 50 and 52 can be used toprovide ninety degrees of rotation of the reflected CAM laser line withrespect to the FTOE and RTOE laser lines. In this way, the ninety degreedifference shown in FIG. 5 between the orientation of the CAM laser lineand that of the FTOE and RTOE laser lines can be eliminated.

By combining the three laser lines into a single image, a plurality ofadvantages are realized. First, only one camera and associatedelectronics are necessary for each sensor. This eliminates the need forcalibrating the positions of multiple cameras within a sensor andeliminates the possibility of erroneous measurements due to errors orchanges in the relative positions of the multiple cameras. Second, thethree laser lines contained in the image can be related to a commonreference, preferably one representing a wheel rotational plane havingzero camber and zero toe. This permits direct comparisons between theposition of the high points of the laser lines with the common referenceand with each other to determine the wheel alignment characteristics.Third, since each sensor only generates a single video image and sincethe three laser lines within that single image are related to a commonreference, real time processing of the camera's video output can beeasily accomplished within the sensor to generate conventionalcoordinate data. Furthermore, the use of optical system 16 to producethe single image seen by camera 18 is advantageous because it providescamera 18 with a view of all three laser lines in the same orientationand without requiring that camera 18 be positioned far enough away fromwheel 28a so as to include all three laser lines within the field ofview of lens 18a.

Camera 18 can be a conventional video camera that utilizes a ccd sensorhaving a rectangular array of pixels. Preferably, it comprises aPhillips FTM12 camera that has a sensor array of 1024×1024 pixels,although it will be understood that other types of suitable lightresponsive receivers could be used, such as a photodiode array or alinescan or rotating polygon linescan camera. Camera 18 is mountedwithin housing 22 in an orientation that is rotated ninety degrees aboutits optical axis from what would otherwise be its normal orientation.This is done so that the parabolic laser lines are seen from thecamera's angle of reference as extending generally in the verticaldirection. More specifically, and as will be apparent from an inspectionof FIG. 5, the parabolically-shaped reflected FTOE and RTOE laser linesenter housing 22 as U-shaped images that extend generally in thehorizontal direction. Thus, the crown, or high point, of each laser linewill be the lowest (i.e., closest to the ground) point on the line. Thisorientation is maintained by optical system 16. The CAM laser lineenters housing 22 in a generally vertical orientation as a C-shapedimage and is thereafter rotated by ninety degrees so that it has thesame the same U-shaped orientation as FTOE and RTOE when it exitsoptical system 16. By rotating camera 18 ninety degrees, its top will beproximate one of the sides of housing 22 and the parabolic laser lineswill be seen by camera 18 as having either a C-shape or backwardsC-shape. As shown in FIG. 9, if camera 18 is rotated such that its top18b is toward the side of housing 22 at which mirrors 50 and 52 arelocated, then the three laser lines will be seen by the camera as eachhaving a backwards C-shape.

This orientation of the FTOE, RTOE, and CAM laser lines is shown in FIG.10, which represents an image 58 outputted by camera 18. In that figure,the three parabolic curves represent the CAM, RTOE, and FTOE laserlines. The labels shown in FIG. 10 for these laser lines are not a partof the image output by camera 18, but rather are included in FIG. 10 forthe purpose of clarity. Similarly, the horizontal scan lines 60, 62, and64 and the vertical reference line 66 shown in that figure are not partof the actual image, but are provided for the purpose of describing howthe high points are determined and used as a part of calculating the toeand camber of wheel 28a. Since electronic circuit 20 outputs a VGA videosignal for display of the camera image, these labels, reference line 66and cursors showing each curves high points can be superimposed over theimage by circuit 20 for the purpose of display.

By orienting the laser lines with respect to camera 18 in this manner,the camera's video signal output can be easily analyzed by electroniccircuit 20. Since the laser lines have a generally vertical orientation,as seen by camera 18, each of a number of successive scan lines of thecamera will intersect one of the three laser lines. Since the laserlines have a backwards C-shape, the high point of each line will be thatpixel on the laser line having the highest column number of any of thepixels on the laser line. This is depicted in FIG. 11 for the CAM curveof FIG. 10. Camera 18 is programmed by electronic circuit 20 to work ina non-standard operating mode. In this mode, camera 18 outputs anon-standard 40 MHz VBS video signal, which is a composite video signalformatted as a stream of pixel data points arranged into groups. Each ofthese groups of pixel data points represents one of the scan lines(rows) of the camera's array of pixels. By detecting, for each scan line(i.e., each row of the array of pixels), the column number of the pixellying on the laser line and comparing that column number to the highestsuch column number found, the pixel representing the high point of thelaser line can be determined. For the RTOE and FTOE laser lines, the rowand column number of this pixel represent its X and Y coordinates,respectively. As the CAM laser line has been rotated ninety degrees withrespect to the other laser lines, the row and column number of the pixelcontaining the CAM laser line's high point represent its Y and Xcoordinates, respectively.

The pixel data points outputted by camera 18 are represented by ananalog signal having a voltage level that varies pixel by pixel toindicate the pixel intensity level. Since the pixels lying on a laserline will have a much greater intensity than pixels lying on either sideof the line (due in part to the filters 36 and 38), detection of a pixelthat is located on the laser line can be accomplished by monitoring thepixel intensity levels. This is carried out by electronic circuit 20which monitors the voltage level of the camera's video signal, as willbe described subsequently.

As mentioned above, the reflected FTOE, RTOE, and CAM laser lines canall be related to a common reference. This can be accomplished bycalibrating the positions of lasers 12-14, optical system 16, and, ifnecessary, camera 18 using a master gauge or buck (not shown) thatrepresents a wheel having zero camber and zero toe. Lasers 12 and 14 arealigned using the buck so that the high points of the resultingparabolic laser lines on the buck both lie within the same horizontalplane. Laser 13 is aligned so that the high point of the resulting laserline on the buck lies within a vertical plane that is midway between thehigh points of the other two laser lines. Optical system 16 and camera18 are then aligned so that the high points of the three laser lines lieon a single vertical line (i.e., have the same pixel column number and,thus, the same X coordinate). As a result of this calibration, the threelaser lines reflected from the buck will produce image 58 shown in FIG.10. The vertical reference line 66 of FIG. 10 thus represents the commonreference to which the three reflected laser lines of any particularwheel can be related and compared.

Determination of the camber and toe angles involves use ofsensor-to-wheel distances z_(f), z_(r), and z_(c) that can be determinedfrom the X and Y coordinates of the laser lines' high points. Asdiscussed above, the use of this coordinate data to determine thesensor-to-wheel distances (and, thus, the camber and toe angles) is wellknown. However, for the purpose of illustrating certain advantages ofsensor 10a, the relationship between the coordinate data andsensor-to-wheel data will now be discussed in connection with FIGS. 12Athrough 13B.

FIG. 12A shows wheel 28a having positive toe and zero camber and FIG.12B shows the resulting image provided by camera 18. As will beappreciated, reference line 66, as it applies to the FTOE and RTOE laserlines, represents the sensor's X-axis and, as it applies to the CAMlaser line, represents the sensor's Y-axis. Reference line 66 istherefore shown in FIG. 12A as having been split into two lines 66a and66b. The positive toe of wheel 28a results in the portion of sidewall32a closest to the front of the vehicle being farther away from sensor10a than the reference line 66b. At the same time, it results in theportion of sidewall 32a farthest from the front of the vehicle beingcloser to sensor 10a than reference line 66b. Since lasers 12 and 14direct their laser light upwards toward wheel 28a, the FTOE laser linefrom laser 12 will fall upon sidewall 32a above reference line 66b andits high point will therefore lie on one side of line 66b. Conversely,the RTOE laser line will fall upon sidewall 32a below reference line 66band its high point will therefore lie on the other side of line 66b. Thehigh point of the CAM laser line lies on the reference line 66a,although this is not necessarily so for zero camber since the verticalaxis about which wheel 28a pivots when toe adjustments are made may notextend exactly through the wheel's center. Also, deviations of the highpoint of the CAM laser line from the line 66a might occur as a result ofthe vehicle not being properly centered and aligned in the test rig.

As a result of the positive toe shown in FIG. 12A, camera 18 produces animage 68 that includes the CAM, RTOE, and FTOE laser lines, as shown inFIG. 12B. The distances y_(r) and y_(f) are the distances along theY-axis between reference line 66 and the high points of RTOE and FTOE,respectively. These distances together are indicative of the wheel's toeangle. Since the angle between the emitted and received laser light isknown, these Y-axis distances can be converted into Z-axis distancesz_(r) and z_(f) shown in FIG. 6. The Z-axis distances can then be usedalong with the symmetry correction to calculate the wheel's toe angle.As will be understood by those skilled in the art, the toe angle can bedetermined by directly converting the total distance y_(t) (i.e., Y_(r)+Y_(f)) between these high points into a Z-axis distance z_(t) andthereafter calculating toe using that distance along with the symmetrycorrection.

FIG. 13A shows wheel 28a having positive camber and zero toe and FIG.12B shows the resulting image provided by camera 18. The high points ofthe FTOE and RTOE laser lines are shown as lying slightly belowreference line 66b. In this example, the offset of those two laser linesfrom the reference line occurs not as a result of any toe angle, butbecause the horizontal axis about which changes in camber are made liesbelow the horizontal plane containing the center of wheel 28a. Thepositive camber results in the uppermost part of sidewall 32a beingcloser to sensor 10a than the lowest part of sidewall 32a. Thus, the CAMlaser line projected by laser 13 contacts sidewall 32a at a location tothe left of reference line 66a.

The resulting image 70 is shown in FIG. 13B. As will be appreciated,since wheel 28a has a zero toe angle, the line extending through thehigh points of the RTOE and FTOE laser lines is parallel to referenceline 66. The distance x_(c) equals the displacement along the X-axis ofthe high point of the CAM laser line relative to reference line 66. Thedistance y_(d) equals the distance between reference line 66 and themidpoint of the line segment extending between the high points of theRTOE and FTOE laser lines. Using the known angles between the emittedand received laser light for lasers 12-14, these distances can beconverted to the Z-axis distances z_(c) and z_(d) that are shown in FIG.7 and that are used along with the symmetry correction to calculate thecamber angle. The distance y_(d) is used as a part of determining camberto account for the possibility that the actual camber angle of wheel 28ais about an axis that does not lie in the horizontal plane containingreference line 66. Further, y_(d) is determined using the line segment'smidpoint to eliminate the effect on the camber calculation of thewheel's toe angle, if any.

Sensor Electronics Overview

Electronic circuit 20 is an event-driven, microprocessor based circuitthat processes the video stream from camera 18 and outputs two types ofinformation: 1) X and Y coordinate data for the locations of the highpoints of each of the three laser lines, and 2) a modified video signalfor displaying the three laser lines on monitor 25. As discussed above,the coordinate data can be processed in a well known manner by computer24 to determine the alignment characteristics of the wheel.Determination of the coordinate data by circuit 20 is carried out inreal time, which is possible with only minimal processing power becauseof the merging of the three laser lines provided by optical system 16.

Before describing the construction and operation of circuit 20 indetail, the format of the raw video signal from camera 18 and anoverview of the method for determining the high point of the FTOE, RTOE,and CAM laser lines will be described. The video signal is an analogsignal having a voltage that varies at 40 MHz in accordance with theintensity levels of the pixels that it represents. The video signalcomprises successive groups of analog pixel data, with each grouprepresenting one of the rows of the array of pixels that make up thecamera's ccd element. An exemplary scan line (row) for this video signalis depicted in FIG. 15 as VIDEO IN. Each scan line of the video signalis separated by a horizontal sync HS, as shown in FIG. 15. Camera 18 isoperated in a non-interlaced mode in which it provides frames containingevery other scan line at a rate twice that of the camera's 30 Hz fullframe rate. However, rather than providing alternating frames of evenand odd-numbered scan lines, camera 18 has its field mode programmed toprovide only frames of even-numbered scan lines. Thus, those frames ofeven-numbered scan lines are outputted by camera 18 at a rate of 60 Hz,with those frames being separated by a vertical sync. The horizontal andvertical syncs, as well as a clock signal at the 40 MHz pixel rate, arealso outputted by camera 18 on separate lines, as indicated by HS, VS,and CK40M, respectively, in FIG. 14.

Determination of the location of the laser lines is accomplished bycomparing the video signal to a threshold level. Intersection of one ofthe laser lines with a particular video scan line is represented in thevideo signal by a voltage level that exceeds that threshold. In thisway, a simple comparator can be used to generate a signal that indicateswhether or not the pixel intensity level being compared represents anintersection between a reflected laser line and one of the video scanlines. At the beginning of each scan line (i.e., at each occurrence ofthe horizontal sync signal HS), a counter is begin that increments atthe 40 MHz pixel rate. If the video signal voltage exceeds thethreshold, this counter is stopped and its count is moved into a latch.If this pixel count represents the high point of the laser line, then itand its corresponding scan line number are utilized by a microprocessorto generate the X and Y coordinate data for that high point.

As discussed in connection with FIG. 11, the high point of anyparticular laser line is represented by the intersection point havingthe highest column number. Determination of whether a particularintersection point represents the laser line's high point can beaccomplished in various ways. The simplest of these is referred tohereafter as single scan line measurement and involves monitoring onlyone scan line per laser line. For this technique, the intersection pointwithin that one scan line is assumed to be the laser line's high point.For each laser line, the single scan line actually monitored can beselected as a part of the initial set-up of sensor 10a using the mastergauge or buck discussed above. More specifically, once lasers 12-14,optical system 16, and camera 18 are properly aligned using the buck,circuit 20 can be commanded to run an initialization routine in which itanalyzes successive scan lines to determine the three scan linescontaining the three high points of the laser lines reflected from thebuck. The row numbers of these three scan lines can then be stored bycircuit 20 and thereafter used in connection with vehicle testing toignore all but those three scan lines when analyzing the video datastream for the three high points. Although the high points of the wheelunder test may not lie exactly on these scan lines, the curvature of thelaser lines is sufficiently small that, as long as the vehicle is notgrossly misaligned with the test rig, the difference in column numbersbetween the actual high points and the intersection points for thestored row numbers will be small and will have little effect on theaccuracy of measurement.

Rather than examining a single scan line for each of the three laserlines, a number of scan lines could be monitored to determine the highpoint. This can be accomplished either by monitoring every scan linesent from camera 18 or by monitoring only a subset of those scan lines.In either case, determination of the high point involves comparing thecolumn number of each intersection point with the highest column numberpreviously found. For each scan line, if the column number of theintersection point is greater than the highest column number previouslyfound, the new column number is stored (along with the row number) forcomparison to that of the next scan line. As will be appreciated, if thecolumn numbers of the intersection point increase for successive scanlines, then the scan line containing the high point of the laser linehas not yet been reached. Conversely, if the column numbers of theintersection points decrease, then the scan line containing theintersection point has been passed. At this point no further scan linesneed be analyzed for that particular laser line and the stored row andcolumn numbers will represent the X and Y coordinates of the high point.To further refine the analysis of the incoming video data stream,windowing can be utilized not only to ignore certain scan lines (rows)located remotely of the high points, but also to ignore a certain numberof columns located remotely on either side of the high points. Thelocation of the window can be determined as a part of the initial set-upusing the master gauge, or can be determined for a particular wheel byinitially locating the high point for each laser line and then forming awindow around that high point and thereafter using the window forsubsequent frames of video data. Preferably, the length and width ofeach window is predetermined.

Sensor Electronics Construction and Operation

FIG. 14 depicts the construction of electronic circuit 20 that providesthe above-described real-time image processing and its operation will bediscussed in connection with the timing diagram of FIG. 15. At the heartof circuit 20 is a microprocessor 80 that controls monitoring of thescan lines and generation of both the video display and coordinate datathat is sent to computer 24. Preferably, microprocessor 80 is an68HC11E2FN, manufactured by Motorola, that has its control programstored internally in ROM. While microprocessor 80 need not be clocked insynchronism with camera 18, its operation is timed with the video datastream at least on a frame by frame basis, and it therefore uses two ofits data inputs to receive the camera's vertical sync signal VS andhorizontal sync signal HS via a sync buffering circuit 82. Themicroprocessor runs at 8 MHz using a clock signal that can either begenerated by a crystal oscillator or derived from the camera's 40 MHzclock CK40M using a system clock circuit 84. The video input signal(VIDEO IN) from camera 18 is provided to an input conditioning circuit86. The output of that circuit is provided to an analog comparator 88where it is compared to a threshold signal VTHRESH that is generated bya digital-to-analog converter 90 using data from microprocessor 80. Foreach of the 40 MHz pixels, the output of comparator 88 indicates whetheror not that pixel lies on one of the laser lines. It outputs a logic onelevel if the pixel lies on a laser line and a logic zero level if not.This output is provided to a gated edge detector 92 that, when enabled,responds to a positive transition from comparator 88 by sending aninterrupt request (IRQ) to microprocessor 80. Circuit 20 furtherincludes a sixteen bit counter 94 having its thirteen least significantbits connected to the corresponding parallel inputs of a sixteen bitlatch 96. Pixel counter 94 is reset at the beginning of each scan lineand is operated by a gated clock signal CK40MG that is generated by edgedetector 92 using the camera's 40 MHz clock CK40M. In addition togenerating an interrupt request, edge detector 92 stops pixel counter 94and latches the pixel counter data into latch 96. This latched datarepresents the pixel count from the beginning of the scan line and,hence, the column number of the pixel detected by comparator 88 as lyingon the laser line. This pixel count is then read by microprocessor 80over a data bus 98 as a part of the interrupt service routine (ISR)initiated by the interrupt request IRQ. To implement the single scanline measurement or windowing discussed above, edge detector 92 isselectively enabled and disabled by an enable signal ENBL that isgenerated by a CFR gating circuit 100 using information obtained frommicroprocessor 80 over data bus 98. CFR gating circuit 100 alsogenerates three signals, CAM, RTOE, and FTOE used as a part of singlescan line measurement to indicate as a part of the pixel count whetherthe intersection point represented by the pixel count is for the CAM,RTOE, or FTOE laser line.

The blocks of FIG. 14 that comprise circuit 20 will now be described ingreater detail, followed by a description of the specific operation ofcircuit 20. Sync buffering circuit 82 is used to provide microprocessor80 with the horizontal and vertical sync signals HS and VS. Using thesesignals, microprocessor 80 can keep track of the scan line to which theincoming pixel data belongs. Sync buffering circuit 82 can also be usedto provide microprocessor 80 with an odd/even signal when camera 18 isoperated in the non-interlaced mode to provide alternating frames of oddand even scan lines. For video cameras such as camera 18 which provideseparate horizontal and vertical sync outputs, sync buffering circuit 82can simply be inverters or other buffers. For cameras that do not havededicated sync outputs, but only provide the syncs as a part of acomposite picture signal, a video sync separator can be used. Forexample, sync buffering circuit 82 could utilize an LM1881 manufacturedby National Semiconductor that receives the VIDEO IN signal and outputsa composite (horizontal) sync, a vertical sync, and if necessary anodd/even signal, all of which can then be provided to microprocessor 80.The horizontal and vertical sync signals are also buffered to provide amonitor horizontal sync MHS and a monitor vertical sync MVS,respectively. These two monitor sync signals are sent to the videomultiplexor 24b of computer 24 to permit synchronization of monitor 25with the video data stream generated by camera 18. The horizontal syncsignal is used to generate an end of line signal which triggers amonostable within circuit 82. This monostable is used to generate theend of line reset pulse EOL shown in FIG. 15 which resets pixel counter94 at the end of each line and holds it at reset until the first pixelof the next scan line. System clock circuit 84 generates the 8 MHzmicroprocessor clock using a conventional crystal oscillator.Alternatively, the microprocessor clock signal can be derived from the40 MHz clock signal CK40M output by camera 18.

Input conditioning circuit 86 utilizes a wideband video amplifier, suchas an LM1201 manufactured by National Semiconductor. It receives aninput contrast signal (ICONTRAST) and an input brightness signal(IBRIGHT) generated by DAC 90 using data from microprocessor 80. Thesesignals are used by the video amplifier to provide suitableamplification and dc offsets to the raw video data stream (VIDEO IN) tobring it to the appropriate levels for comparison to the thresholdvoltage VTHRESH.

Comparator 88 can be a standard voltage comparator, such as an LM 311manufactured by National Semiconductor. It receives the conditionedVIDEO IN signal generated by conditioning circuit 86 as one input andthe threshold voltage VTHRESH from DAC 90 as its other input. As shownin FIG. 15, its output, TVO, is at a logic zero level when VIDEOIN<VTHRESH and changes to a logic one level when VIDEO IN crossesVTHRESH.

Gated edge detector 92 receives the TVO signal generated by comparator88. When enabled by CFR gating circuit 100, edge detector 92 responds totransitions of the TVO signal to do several things: 1) it generates theinterrupt request IRQ to inform microprocessor 80 that an intersectionpoint has been found, (2) it stops counter 94 to hold the pixel count atthe column number within which the intersection point is located, and(3) it latches that pixel count into latch 96. Edge detector 92 can beimplemented in part using a CD4538 or other suitable monostablemultivibrator that is triggered by positive transitions of the TVOsignal. Edge detector 92 is enabled or disabled by gating the output ofthis monostable with the enable signal ENBL. When edge detector 92 isenabled, the gated output pulse can then be used to generate theinterrupt request signal IRQ and to trigger a second monostable thatgenerates a LATCH signal that latches the pixel count data into latch96. The ENBL, IRQ, and LATCH signals are shown in FIG. 15 and theirtiming and use will be further described below. It will of course beunderstood that microprocessor 80 could be interrupted using one of itsdata or other inputs configured to recognize logic transitions, ratherthan utilizing a dedicated interrupt request input. Also, microprocessor80 could be configured to monitor one of its data inputs for a logiclevel that indicates detection of an intersection point. Thus, as usedherein, the term "interrupt input" will be understood to refer to eithera dedicated interrupt input or any other microprocessor input that canbe used to inform microprocessor 80 of the detection of a pixel on oneof the laser lines.

The gated clock CK40MG that is used to increment counter 94 can begenerated by gating the camera's CK40M with the inverted output of thissecond monostable so that, as soon as it is triggered, the CK40M clocksignal is inhibited from further incrementing counter 94. If it isdesirable to detect the end of the intersection between a laser line andscan line, then negative going transitions of TVO can be detected byedge detector 92 as well. This could be done by providing TVO to anegative edge triggered monostable whose output is OR-tied with theoutput of the first monostable prior to that output being gated with theENBL signal.

Pixel counter 94 can be implement using two, eight bit counters 94a and94b, which can each be implemented using a 74HCT393 dual, four-bitbinary ripple counter configured as an eight-bit counter. The MSB ofcounter 94a is connected as the clock input of counter 94b to provide asixteen bit counter. As indicated above, the first clock input ofcounter 94a receives the gated 40 MHz clock CK40MG and counters 94a and94b have their reset inputs connected to receive the end of line resetpulse EOL so that they are reset at the beginning of each new scan line.The output of counters 94a and 94b are provided to latch 96.

Latch 96 comprises a pair of eight-bit latches 96a and 96b, with latch96a receiving pixel count data from counter 94a and latch 96b receivingpixel count data from counter 94b. These latches provide temporarystorage of the pixel count until it is read by microprocessor 80 viadata bus 98. They can be implemented using 74HCT373 tri-state, octalD-type latches. As indicated above, the clock inputs of these twolatches receives the LATCH signal from edge detector 92. Since, in theillustrated embodiment, data bus 98 is only eight bits wide, only eightbits of the pixel count are read at a time by microprocessor 80 and theoutput enable (OE) inputs of latches 96a and 96b must therefore beasserted sequentially rather than simultaneously. As will be understoodby those skilled in the art, this can be accomplished by assigning aunique address to each latch and then connecting a 74HCT138 or othersuitable decoder to the address lines of microprocessor 80. In this way,microprocessor 80 can select latch 96a, latch 96b, or any other chipconnected to the data bus by providing its associated address on themicroprocessor's address lines.

CFR gating circuit 100 is used to selectively enable and disable edgedetector 92. In particular, edge detector 92 is disabled (1) during theoccurrence of those pixel data points for which monitoring is notdesired and (2) following an interrupt request by edge detector 92.Single scan line measurement is achieved by disabling edge detector 92during every scan line except those three selected by microprocessor 80as containing a different one of the high points of the three laserlines. Windowing is achieved by disabling edge detector 92 during thosescan lines that are to be completely ignored and during occurrence ofthose pixel data points of the remaining scan lines that are outside thewindow. Thus, for example, for a window around the CAM laser line thatextends from row (scan line) 250 to row 350 and from column 500 tocolumn 700, edge detector 92 would be disabled for the even-numberedrows before 250 and after 350 and, for the rows from 250 to 350 would bedisabled for the columns (pixel data points) within each row that arebefore 500 and after 700. Thus, microprocessor 80 could only beinterrupted in response to the pixel intensity level of a pixel datapoint located within this window.

CFR gating circuit 100 can be constructed in a variety of ways todisable edge detector 92 during the appropriate times. Preferably, thisis accomplished using three video sync separators (one per laser line)that are programmed via data bus 98 by microprocessor 80 to generateblanking signals during those scan lines and/or pixel data points thatare to be ignored. The blanking signals output by these three syncseparators can be combined to form the enable signal ENBL that disablesedge detector 92. These three sync separators can be implemented usingthree LM 1882 video sync separators, manufactured by NationalSemiconductor, and the programming necessary to generate the blankingsignals is well within the level of skill in the art. These syncseparators can be updated as necessary by microprocessor 80 once eachframe (i.e., following each vertical sync VS), although this updateinformation is not necessary if single scan line measurement isutilized. For each of the three sync separators, the update informationcan be determined during the previous video frame as a part of theinterrupt service routine executed for its associated laser line. Thatinformation can then be stored in temporary memory, such as an externalRAM, until used by microprocessor 80 to update the sync separators.Optionally, if edge detector 92 is only to be enabled or disabled on arow-by-row basis, these three sync separators would not be required.Rather, microprocessor 80 could simply provide a single bit that isupdated at the beginning of each scan line and that specifies whether ornot edge detector 92 should be disabled during that scan line. This bitcould then be provided directly to edge detector 92 to gate the TVOsignal and CFR gating circuit 100 would therefore not be needed.

Disabling of edge detector 92 following an interrupt request isaccomplished using an inhibit signal (INH) output from microprocessor80. As discussed in greater detail below, the interrupt request sentfrom edge detector 92 to microprocessor 80 causes the microprocessor tobegin execution of an interrupt service routine. The inhibit signal INHis asserted during this interrupt service routine to prevent theoccurrence of additional interrupts while the interrupt service routineis being executed. This is shown in FIG. 15. The inhibit signal INH canbe combined with the blanking signals as a part of forming the enablesignal ENBL used to enable and disable edge detector 92. For anyparticular scan line, the first pixel data point having an intensitylevel above the threshold (and, thus, the first interrupt generatedduring that scan line) is assumed to represent the intersection of thatscan line with one of the three laser lines. However, other tests forthe intersection point could be made. For example, once the firstinterrupt on a monitored scan line is generated, a small number ofconsecutive pixel data points could be monitored as a way of verifyingthat those pixel data points represent the intersection of one of thelaser lines with the current scan line. Other such variations willbecome apparent to those skilled in the art.

As discussed above, for single scan line measurement, microprocessor 80determines which three scan lines are to be monitored and provides thisinformation to CFR gating circuit 100 so that edge detector 92 can bedisabled for all but those three lines. When an interrupt is generatedfor a scan line being monitored, microprocessor 80 needs to know whichof the three scan lines is associated with that pixel count. This can beaccomplished using microprocessor 80 to count the number of horizontalsyncs HS occurring since the last vertical sync VS. When the interruptoccurs, the current count of horizontal syncs will indicate the scanline to which the pixel count belongs. Another way to determine the scanline associated with the pixel count would be to reset a counter at thebeginning of each frame and increment the counter once for everyinterrupt. Then, the number in the counter (1, 2, or 3) will indicatewhich one of the three scan lines is associated with the pixel count.

Yet another way to determine the scan line is to place data indicativeof the current scan line into the upper, unused bits of the pixel count.More specifically, since camera 18 has 1024 pixels per scan line (row),the pixel count provided by counter 94 will not exceed 1024 and only tenbits of counter 94 are therefore needed. This leaves the six mostsignificant bits of counter 94b available for storing other data. Threeof these unused bits can be used in single scan line measurement tostore with the pixel count data indicative of the scan line for whichthe pixel count relates. This is accomplished by using CFR gatingcircuit 100 to generate three signals, one for camber (CAM), one for theforward toe (FTOE), and one for the rearward toe (RTOE). These signalsare connected to the three most significant bits of latch 96b instead ofthe corresponding bits from counter 94b. Each of these signals isasserted when its associated scan line is being monitored. Thus, onlyone of these signals is asserted at a time and its asserted state willbe latched into latch 96b along with the upper bits of the pixel count.Microprocessor 80 can thereafter check these three most significant bitsto determine which of the three scan lines is associated with the pixelcount.

Electronic circuit 20 also includes a serial I/O circuit 102 that isconnected to the SCI port 80a of microprocessor 80, as well as to camera18 by an RS232 serial line and to serial card 24a of computer 24 by anRS422 serial line. These serial lines can be used to transferinitialization commands and programming between computer 24,microprocessor 80, and camera 18. For example, the RS232 serial line canbe used to set the shutter speed, gain, and other programmableparameters of camera 18. The RS422 serial line is used to transfer thecoordinate data from sensor 10a to computer 24 for determination of thecamber and toe.

Circuit 20 further includes an output conditioning circuit 104, a CFRcursors circuit 106, a set of display drivers 108, and if desired anon-screen display circuit 110. These circuits are used to generate animage for display that includes the CAM, RTOE, and FTOE laser lines anda cross-hair cursor at the high point of each of the laser lines. Outputconditioning circuit 104 receives the TVO signal output by comparator88. As will be appreciated, the TVO signal represents a one bitintensity data point--either zero or full intensity. Conditioningcircuit 104 utilizes the same video amplifier used by input conditioningcircuit 86 and applies an output contrast signal (OCONTRAST) and anoutput brightness signal (OBRIGHT) that are generated by microprocessor80 and converted to an analog signal by DAC 90. These conditioningsignals are used to provide the proper dc offsets andamplification/attenuation to the TVO signal so that it is suitable fordisplay on monitor 25. The conditioned TVO signal is provided to displaydrivers 108 that generate the R,G,B or other signals needed by monitor25.

Superimposed on the image provided by output conditioning circuit 106are the cursors provided by CFR cursors circuit 106. These cursors canbe generated using the three video sync separators within CFR gatingcircuit 100 to generate additional blanking signals that are combined inCFR cursors circuit 106 to generate the cross hairs. The sync separatorsare programmed to generate the cursor blanking signals at the beginningof each frame and, therefore, the cursors are generated at the locationsof the high points determined during the previous frame. The output ofcursors circuit 106 is summed with the output of conditioning circuit104 and the resulting signal represents a single image containing thethree laser lines with a cross hair cursor located at the high point ofeach of the curves.

FIG. 15 depicts various exemplary waveforms and their timing for asingle monitored scan line. In single scan line measurement, this singlescan line would be one of the three scan lines determined during initialcalibration as containing one of the three laser lines' high points. Forthe purposes of illustration, this single scan line will be taken to bethe scan line containing the high point of the CAM laser line and theCAM signal generated by CFR gating circuit 100 is therefore included inFIG. 15.

When camera 18 and circuit 20 are initially powered up, camera 18 isinitialized to set its various programmable features (e.g., shutterspeed, field mode) as desired. This programming is provided via theRS232 line and can be provided either by microprocessor 80 operatingunder its control program, or by computer 24. Microprocessor 80 alsooutputs the ICONTRAST, IBRIGHT, OCONTRAST, and OBRIGHT signals to DAC90, which converts them to analog form and provides them to conditioningcircuits 86 and 104. Microprocessor 80 thereafter begins processing thevideo data outputted by camera 18.

At the beginning of a frame, microprocessor 80 updates the video syncseparators of CFR gating circuit 100, if necessary, with the new scanline and cursor information. The ENBL signal is kept unasserted by thethese sync separators and the INH output of microprocessor 80 until thebeginning of the scan line that was determined during calibration ascontaining the high point of the CAM laser line. While the ENBL signalis unasserted, transitions of TVO due to the conditioned VIDEO IN signalcrossing the threshold signal VTHRESH do not result in generation of aninterrupt request IRQ. At the beginning of each scan line, thehorizontal sync HS causes generation of the end of line reset pulse EOLthat resets pixel counter 94. Then, counter 94 begins counting insynchronism with the incoming pixel data points for the current scanline. For scan lines not being monitored, the pixel counter will keepcounting until it is reset by the next EOL pulse.

At the beginning of the CAM scan line, the CAM signal from CFR gatingcircuit 100 is asserted, causing the ENBL signal to be asserted andthereby enabling edge detector 92. At this point, any positivetransitions of TVO will result in generation of an interrupt request IRQthat begins the microprocessor's interrupt service routine. Thus, asshown in FIG. 15, when the voltage of the conditioned VIDEO IN signalcrosses the threshold voltage VTHRESH, TVO goes high causing generationof the interrupt request IRQ, as well as the LATCH signal that latchesthe pixel count into latch 96. At the same time, the gated 40 MHz clockCK40MG used to increment counter 94 is disabled. During execution of theinterrupt service routine, the INH output of the microprocessor isasserted, causing the ENBL signal to become unasserted. This disablesedge detector 92 from generating further interrupts, such as wouldotherwise occur as a result of the positive transition of TVO shown at(a) in FIG. 15.

As a part of the interrupt service routine, microprocessor 80 reads thepixel count from latch 96 and associates it with the scan line to whichit belongs. Once all three high points have been found for a particularframe, microprocessor 80 uses the stored pixel counts and scan line(row) numbers to generate the coordinate data to be sent to computer 24.This coordinate data can be in the form of raw data (pixel count andscan line number) or can be converted to scaled engineering units, asdesired for a particular application.

The combination of arranging the three laser lines into a single imageand the use of a comparison circuit to detect the intersection pointbetween a laser line and video scan line provides a significantadvantage over prior art non-contact optical wheel alignment sensors.Not only does this combination of features permit real-timedetermination of the spatial positions of the three points used todefine the rotational plane of the wheel, but also it permits the sensorto output conventional data that can be processed using widely knowntechniques for determining the wheel alignment characteristics.

Rotation of the Vehicle Wheel's During Measurement

As is known, measurement of a wheel's camber and toe angle is preferablyundertaken while the wheel is rotated on its axle. Measurement of thesealignment characteristics by sensor 10a while the wheel is rotating hascertain advantages. First, kinematic effects of the rotating wheel(e.g., play in the ball-bearings) will be reflected in the measurement.Second, rotation of the wheel during measurement permits surfaceirregularities on the tire sidewall to be averaged out or evencompletely eliminated. For example, raised lettering on the tiresidewall that affects the position of the FTOE high point will have thesame affect on the position of the RTOE high point once the wheel hasrotated 180°. Since the toe measurement is determined in accordance withthe difference in position of the high points of the RTOE and FTOE laserlines, these two effects will offset one another and the effects of theraised lettering will therefore be averaged out over the course of afull revolution.

For any particular wheel alignment application, the speed of rotation ofthe wheel is limited only by the number of points required perrevolution. To achieve a data point every two degrees of rotation, thewheel would be rotated at a rate of one revolution every three seconds,giving 180 data points per revolution for a video frame rate of 60 Hz.Preferably, the coordinates of the high points of the three laser linesare averaged over the course of 90° of rotation and this average data issent to computer 24 along with or in lieu of the instantaneouscoordinate data. This can be implemented under control of microprocessor80 using an array that stores high point locations and then averagesthose values once the array is full. The array can have a variable sizedetermined in accordance with the speed of rotation so that the arraybecomes full at the completion of each 90° of rotation of the wheel.

Other Applications

As should now be apparent, since the rotational plane of the wheels canbe determined using the sensors 10a-10d, other wheel alignmentcharacteristics, such as the caster and steering axis inclination (SAI)angles, can be determined. One technique for determining these angles isdescribed in U.S. Pat. No. 5,291,660, issued Mar. 8, 1994 to A. Koerner,the disclosure of which is hereby incorporated by reference. That patentdiscloses techniques for determining the caster and SAI angles ofsteerable wheels in accordance with measured coordinate displacements ofthe wheel when the wheel is steered left and right through the sameangle. As will be appreciated, the sensors of the present inventioncould be used to generate coordinate data from which the displacementscould be determined. These displacements could then be used with themeasured steered angle to determine the caster and SAI angles, as isexplained in detail in that patent.

Furthermore, although sensor 10a has been described as it might be usedin a wheel alignment machine, it will be appreciated by those skilled inthe art that, in the broader aspects of the invention, the sensor of thepresent invention can be used to determine the position, orientation, orother spatial attribute of any of a number of objects other than vehiclewheels and that its application is therefore not limited to wheelalignment. Since the sensor method and apparatus of the inventionpermits real-time analysis of one or more such spatial attributes, itcan be used in such applications as: robotics or other automationcontrol to track a moving target; real-time object distance, angle,and/or position detection; and real-time visual guidance control. Also,it could be utilized for analyzing speed, motion, and/or vibration ofone or more objects. Other optical devices could be used in addition toor in lieu of optical system 16 to provide the field of view,orientation, resolution, or other optical characteristic desired for theparticular application. For instance, bore scopes, fiber optics, andanamorphic lenses or mirrors could be used to provide the desired objectorientation and perspective.

It will thus be apparent that there has been provided in accordance withthe present invention an optical sensor method and apparatus whichachieves the aims and advantages specified herein. It will of course beunderstood that the foregoing description is of a preferred exemplaryembodiment of the invention and that the invention is not limited to thespecific embodiment shown. Various changes and modifications will becomeapparent to those skilled in the art. For example, in the illustratedembodiment, rather than using mirrors within optical system 16, bundlesof fiber optics could be used to receive the three reflected laserlines, rotate the CAM laser line, and then combine the three lines intothe single image seen by camera 18. All such variations andmodifications are intended to come within the scope of the appendedclaims.

We claim:
 1. In a sensor for use in a wheel alignment machine to measurethe orientation of a tire on a vehicle to thereby determine one or morealignment characteristics of the vehicle, said sensor being of the typehaving at least one light source oriented to project shaped light onto asidewall of the tire at a plurality of spaced locations and a lightresponsive receiver oriented at a perspective angle with respect to saidlight source to receive an image that includes a portion of the shapedlight that is reflected off the tire, with said light responsivereceiver being operable to generate electrical signals indicative of theimage, wherein the improvement comprises:a system of optical elementsoriented relative to said light responsive receiver to provide saidlight responsive receiver with an optical view that includes theplurality of spaced locations of the tire, whereby portions of theshaped light that are reflected off the tire at each of the plurality ofspaced locations are received by said light responsive receiver as asingle image; and an electronic circuit responsive to said electricalsignals to determine the location within the image of a predeterminedfeature of each of the reflected portions of shaped light, said circuitfurther being operable to generate output data representative of thelocations of said predetermined features.
 2. A sensor as defined inclaim 1, wherein said optical elements are oriented to rotate at leastone reflected portion of the shaped light with respect to at leastanother reflected portion of the shaped light.
 3. A sensor as defined inclaim 2, wherein said at least one light source comprises first, second,and third lasers, each of which is oriented to project a stripe of laserlight onto the sidewall at a different one of the plurality of spacedlocations, whereby the shaped light at each of the plurality of spacedlocations comprises a stripe of laser light.
 4. A sensor as defined inclaim 3, wherein said first and second lasers are oriented to projectlight in substantially parallel planes and said third laser is orientedto project light in a plane that is substantially perpendicular to saidparallel planes.
 5. A sensor as defined in claim 3, wherein:said lightresponsive receiver comprises a video camera having an image receivingelement that includes successive scan lines, each of which comprises anumber of pixels; said video camera is oriented to receive the reflectedportions of the stripes of laser light as lines of laser light thatintersect at least some of said scan lines; said video camera isoperable to generate said electrical signals as a stream of pixel datapoints arranged into successive lines of said pixel data points, witheach of said lines of pixel data points representing one of said scanlines; said electronic circuit includes a microprocessor and is operableto monitor said stream of pixel data points as it is received from saidvideo camera and to interrupt said microprocessor when said electroniccircuit receives a particular pixel data point representative of any ofthe reflected portions of the stripes of laser light; and saidmicroprocessor is operable in response to said interrupt request toacquire a pixel count representing the position of said particular pixeldata point within its associated line of pixel data points.
 6. A sensoras defined in claim 1, wherein:said light responsive receiver comprisesa video camera that generates said electrical signals as a stream ofpixel data points arranged into successive lines of said pixel datapoints, with each of said lines of pixel data points representing onerow of an array of pixel data points that together represent the image;said electronic circuit is operable to monitor said stream of pixel datapoints as it is received from said video camera and provide saidmicroprocessor with an interrupt request when said electronic circuitreceives a particular pixel data point representative of any of thereflected portions of the shaped light; and said microprocessor isoperable in response to said interrupt request to acquire a pixel countrepresenting the position of said particular pixel data point within itsassociated line of pixel data points.
 7. A sensor as defined in claim 1,wherein said optical elements include a plurality of mirrors oriented todirect the reflected portions of the shaped light into said lightresponsive receiver.
 8. A sensor as defined in claim 1, wherein saidcircuit is operable to generate coordinate output data indicative of thelocation of a preselected feature of each of the reflected portions ofshaped light.
 9. A method for generating data indicative of theorientation of a tire on a vehicle, comprising the steps of:(a)projecting shaped light onto a sidewall of a tire at a plurality ofspaced locations, (b) receiving an image that includes portions of theshaped light reflected at an angle off said sidewall from each of saidplurality of spaced locations, (c) generating a video signal thatcomprises a stream of pixel intensity levels arranged into successivegroups of said pixel intensity levels, each of said groups representinga row of an array of pixels intensity levels that together representsaid image, (d) providing a threshold intensity level that is less thanthose of said pixel intensity levels that represent said reflectedportions of the shaped light, (e) determining a plurality of pixelcounts by repeating the following steps (e1) through (e3) for each of aplurality of said groups of pixel intensity levels:(e1) comparing, inreal time, said threshold intensity level with at least some of thepixel intensity levels of one of said groups, (e2) generating a logicsignal when one of said pixel intensity levels exceeds said thresholdintensity level, and (e3) determining, in response to an occurrence ofsaid logic signal, one of said pixel counts in accordance with theposition of said one of said pixel intensity levels within said one ofsaid groups, and (f) using said pixel counts to generate output datarepresentative of the spatial position of said plurality of spacedlocations.
 10. The method of claim 9, wherein:step (c) further comprisesgenerating said video signal as an analog signal having a voltage levelthat varies in accordance with each of said pixel intensity levels, andstep (e1) further comprises using a comparator to compare said thresholdintensity level and said pixel intensity levels.
 11. The method of claim9, further comprising carrying out step (e3) only once for each of theplurality of spaced locations.
 12. The method of claim 9, furthercomprising the step of using the pixel counts to determine the locationof a predetermined feature of each of the reflected portions of theshaped light.
 13. The method of claim 12, further comprising generatingsaid output data in accordance with the locations of the predeterminedfeatures.
 14. In a sensor for use in a wheel alignment machine tomeasure the orientation of a tire on a vehicle to thereby determine oneor more alignment characteristics of the vehicle, said sensor being ofthe type having at least one light source oriented to project shapedlight onto a sidewall of the tire at a plurality of spaced locations anda light responsive receiver oriented at a perspective angle with respectto said light source to receive an image that includes a portion of theshaped light that is reflected off the tire, with said light responsivereceiver being operable to generate electrical signals indicative of theimage, wherein the improvement comprises:an event-driven microprocessorcircuit connected to receive said electrical signals from said lightresponsive receiver, said circuit including a microprocessor having aninterrupt input; and a system of optical elements oriented relative tosaid light responsive receiver to provide said light responsive receiverwith an optical view that includes the plurality of spaced locations ofthe tire, whereby portions of the shaped light that are reflected offthe tire at each of the plurality of spaced locations are received bysaid light responsive receiver as a single image; said circuit beingoperable, in response to detecting in real time the presence within theimage of the reflected portion of the shaped light, to generate aninterrupt request on said interrupt input.
 15. A sensor as defined inclaim 14, wherein:said at least one light source comprises a pluralityof lasers, each of which is oriented to project a stripe of laser lightonto the sidewall at a different one of the plurality of spacedlocations with the stripes of laser light extending radially withrespect to the tire, whereby the portion of the shaped light reflectedfrom each of the plurality of spaced locations comprises a stripe oflaser light; one of said lasers being oriented to project a first stripeof laser light in a first plane and another of said lasers beingoriented to project a second stripe of laser light in a second planethat forms an angle with said first plane; and said optical elements areoriented to cause rotation of the reflected portion of the first stripeof laser light relative to the reflected portion of the second stripe oflaser light such that the reflected portions of the first and secondstripes of laser light enter said light responsive receiver having thesame orientation.
 16. In an optical sensor for generating dataindicative of a spatial attribute of an object, said sensor being of thetype having at least one light source oriented to project shaped lightonto the object at a plurality of spaced locations and a lightresponsive receiver oriented at a perspective angle with respect to saidlight source to receive an image that includes a portion of the shapedlight that is reflected off the object, with said light responsivereceiver being operable to generate electrical signals indicative of theimage, wherein the improvement comprises:an event-driven microprocessorcircuit connected to receive said electrical signals from said lightresponsive receiver, said circuit including a microprocessor having aninterrupt input; and a system of optical elements oriented relative tosaid light responsive receiver to provide said light responsive receiverwith an optical view that includes the plurality of spaced locations ofthe object, whereby portions of the shaped light that are reflected offthe object at each of the plurality of spaced locations are received bysaid light responsive receiver as a single image; said circuit beingoperable, in response to detecting in real time the presence within theimage of the reflected portion of the shaped light, to generate aninterrupt request on said interrupt input.